1. Field of the Invention
The present invention generally relates to microscopic acoustic measurements, especially the measurement of phase differences in reflected acoustic waves to define distances, velocities, or surface characteristics with very high accuracy.
2. Description of the Prior Art
In 1975 R. A. Lemmons and C. F. Quate invented the scanning acoustic microscope. In this device a high frequency plane wave is focused by an acoustic lens to scan an object located at the focal point of the lens. The acoustic waves either are transmitted through or reflected by the object. In either case, the acoustic waves are thereafter recollimated by a second acoustic lens and are detected with a piezoelectric detector. The detected signals are applied to an oscilloscope to provide a visual display of the object. This device is further described in the Lemmons and Quate patent entitled Acoustic Microscope, U.S. Pat. No. 4,028,933 issued June 14, 1977. In addition, an apparatus of this type is described in the article, "Acoustic Microscopy: Biomedical Applications", by Lemmons and Quate in Science, 188, pp. 905-911, May 30, 1975, and in the article, "Acoustic Microscopy at Optical Wavelengths", by V. Jibsom and C. F. Quate, Applied Physics Letters, 32 (12) pp. 789-791, June 15, 1978. A more complete collection of articles discussing acoustic microscopy can be found in the preamble of U.S. Pat. No. 4,267,732.
A variety of efforts have been made to utilize such apparatus to analyze the surface structure of materials. For example, in a Quate patent U.S. Pat. No. 4,267,732, the apparatus there includes devices for exciting an object of interest so that acoustic waves are propagated from the object. A wave detector and the object are moved with respect to each other in a raster scan pattern so that a visual image can be obtained. In addition, the frequency of the exciting radiation is varied so that the absorption spectra and the Raman frequency mode of the object can be determined.
In a further example, wherein the surface structure of semiconductors and integrated circuits are analyzed for defects, the integrated circuit may be energized by a pulsating electric current with the resistive heating of the circuit being observed. In another embodiment, light propagated through the back of the semiconductor wafer and the resulting acoustic waves propagated through the front surface are imaged and measured.
It is the general objective of the present invention to improve the accuracy of surface measurement techniques in the field of acoustic microscopy by providing a method of, and apparatus for acoustic imaging which uses signals transmitted from a transducer to measure the surface characteristics of a material by establishing the phase shift of the return signals from the material relative to some reference signal.
It is a further object of the invention to use the reflected signal return to accurately measure and maintain constant the distance between the transducer and the object surface to be studied. In a preferred embodiment of this invention, the material surface to be studied is located at the focal plane of the lens.
It is yet another object of the invention to provide pulse transmitting, and receiving means and analyzing means for developing and analyzing the Rayleigh wave characteristics of the material, especially by measuring phase shift in the signal returns from a material substrate located within the focal plane of the transducer.
It is another object of the invention to provide a new structural form of transducer for transmission of at least two separate signals to be used to determine the distance from the transducer to the object surface and/or the wave perturbation along the object's surface. These and other objects are achieved by an acoustic microscope comprising a transducer for transmitting acoustic signals towards the surface to be studied, and means for receiving at least one reflected signal from the surface; in many embodiments of the invention, signals are received from two separate points. The signals received are passed to a synchronous phase detection system for analysis. The signals may be received at the same phase detector input and separated according to their expected time of receipt relative to their time of transmission, or they may be received at separated points on the transducer related to their separated points of transmission. The separated return signals are compared on the basis of their phase differential either to each other or to an internally generated reference signal to analyze the surface characteristics of the material.
For example, minute changes in surface profile or depth profile on the surface of the material can be identified by analysis of the phase shift of the reflected return wave relative to the transmitted wave. In another mode of operation, the velocity of Rayleigh waves traveling along the surface is measured by measuring phase differences of acoustic waves. The relative velocity or phase differential between the originated surface wave and a reference signal is a direct indication of the constituency of the surface through which the waves pass. The waves will travel at different speeds if traveling through a film that adheres well to a surface or one that is somewhat separated from the surface, thereby giving important indicators on data that is highly desirable in the manufacture of devices wherein thin layers are laid down one atop the other, as in the manufacture of integrated circuits.
As a further improvement over known systems for acoustic measurement, a phase lock loop control mechanism is incorporated in the measurement system to establish and maintain a constant distance between the lens and the substrate. This improvement is necessary because the measured phase of the reflected signals is affected by the distance between the acoustic lens and the substrate, which may be changed by ambient temperature drift and/or minor topographical changes in the sample surface. With the addition of the phase lock loop distance control system described in detail below, the phase measured is truly due to the surface material property with no artifacts introduced by topographical variation. Alternatively, this phase lock loop distance measuring system may, when combined with surface scanning means, be used as an unusually accurate way of measuring topographical variations in the surface.
The measurement technique described herein is much more direct and orders of magnitude more accurate than existing techniques based on obtaining the so-called V(z) curve as the material signature.
As a further refinement in the system, a new transducer is described herein incorporating separate electrodes, i.e., a center electrode for the on-axis transmission and reception and an outer concentric ring electrode for the generation and detection of signals which comprise the Rayleigh path signals. The ring electrode may alternatively be divided into segments, with opposite ones wired together in order to propagate waves in a number of different directions. With this modification the system comprises a scanning acoustic microscope ideally suited for analyzing surface properties in two dimensions. In a further embodiment, the ring electrode may be used solely to transmit and receive the distance-measuring reference signals.